An Evolved Packet System (EPS) is new mobile communication network architecture defined by the 3rd Generation Partnership Project (3GPP) under the instruction of Long Term Evolution (LTE). The new architecture is flatter than the prior General Packet Radio Service (GPRS) network and the Universal Mobile Telecommunications System (UMTS) and reserves only the Packet Switched (PS) domain.
However, because many services of operators are provided in the Circuit Switched (CS) domain, to implement the existing CS services over an LTE network, a method for connecting to the CS domain through an EPS network is proposed.
As shown in FIG. 1, a Packet Data Network Gateway (PDN-GW) in the EPS network is connected to a Mobile Switching Center (MSC) in the conventional CS domain through an Interworking Function (IWF), where the IWF is capable of simulating the Iu-CS interface or A-interface, for example, through configuration of the protocol stack of the Iu-CS interface or A-interface. When the UE intends to communicate with the MSC, the UE sends a message to the IWF and the IWF may convert the received message into the signaling format of the Iu-CS interface or A-interface and send the converted message to the MSC. Therefore, the communication between the UE and the MSC is implemented. Similarly, the IWF may forward a message received from the MSC to the UE. In this communication mode, the LTE network serves as an interactive channel between the UE and the IWF.
In the above network architecture, the UE exchanges signaling and data with the IWF through a logical interface Z1, which can be understood as a reference point or an Internet Protocol (IP) path. The signaling or data exchanged between the UE and the IWF is carried over a user plane tunnel of the EPS network. The signaling and data are transferred between the UE, an evolved NodeB (eNodeB), a Serving Gateway (S-GW) or a PDN-GW, and the IWF.
This shows that the signaling or data exchanged between the UE and the IWF will not pass through a Mobility Management Entity (MME) in the architecture, which means that the interaction between the UE and the IWF is transparent to the MME in the network architecture.
Consequently, the handover from an LTE network to a GSM/EDGE Radio Access Network (GERAN) or UMTS Terrestrial Radio Access Network (UTRAN) might fail, where GSM is short for Global System for Mobile communications, and EDGE is short for Enhanced Data rate for GSM Evolution.